Digital camera modules are used in a variety of consumer, industrial and scientific imaging devices to produce still images and/or video. Applications of digital camera modules include image-based recognition applications such as barcode scanning and iris recognition. A camera for such applications may include an imaging lens with relatively large depth of field compared to conventional lenses. Such a large depth of field enables a device using the camera to recognize an object to be relatively insensitive to the object's distance from the imaging lens.
For a fixed imaging lens focal length, the depth of field of the imaging lens is approximately linearly proportional to the lens's f-number N, where N is the ratio of the lens's effective length to its entrance pupil diameter D. See, for example, The Manual of Photography, 9th ed. by Jacobson et al, Focal Press, 2000. The field of view 2α of a camera with an imaging lens having focal length f and an image sensor with diagonal length d is
      2    ⁢    α    =      2    ⁢                  arctan        ⁡                  (                      d                          2              ⁢              f                                )                    .      Expressed in terms of f-number N=f/D,
      α    =          2      ⁢              arctan        ⁡                  (                      d                          2              ⁢                              D                ·                N                                              )                      ,which illustrates that for a constant entrance pupil diameter D, field of view α decreases as f-number N increases. Since depth of field is approximately linearly proportional to the lens's f-number N, field of view 2α also decreases as depth of field increases.
Image-based recognition devices require a camera module having a lens with a smaller field of view (FOV) than lenses in conventional camera modules, while producing images with line-width resolution minimally reduced compared to images formed by conventional camera modules.
Conventional narrow-FOV camera modules achieve a small point of view while maintaining image quality of a larger FOV camera by employing telescope-like compound lenses that include several optical surfaces. A disadvantage of such camera modules is that the manufacturing cost of a compound lens increases with number of optical surfaces.